Salt Effect on the Interaction of 22,24-Diprotonated 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin with a β-Sheet Structure of a Zwitterionic Polypeptide

The effect of sodium chloride on the absorption and CD spectra of the β-sheet structure of a zwitterionic poly(Glu–Val–Lys–Val)/22,24-diprotonated 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin system in aqueous solution was studied under various conditions. The monomeric TPPS was aggregated by the addition of the polypeptide and/or NaCl; at least four kinds of TPPS aggregation were observed. Although the aggregated species with an absorption band at around 489 nm formed an S-chiral conformation at [P]/[D] from 1 to 5 below 100 mM NaCl, the S-chirality changed to R-chirality by increasing to 500 mM NaCl or above. These results may indicate that an increase in the ionic strength induces a chiral conformational transition due to the prevention of electrostatic interactions between TPPS and the polypeptide; also the S-and R-chiralities could be caused by the secondary structure of the polypeptide and original property of TPPS, respectively. It is presumed that the electrostatic interactions play an important role in binding TPPS with the polypeptide as a major driving force. However, even 1000 mM NaCl could not completely shield the electrostatic interactions.